In the musty, ever-expanding wardrobe department that is British period drama, Lady Macbeth stands out like a PVC minidress. Adapted from a 19th‑century Russian novel (and nothing to do with Shakespeare), the film is stripped-back costume drama, with minimal dialogue, music and furniture, and none of the pomp and politeness the genre usually entails. Its heroine, Katherine, wears fetching frocks, admittedly, but having been sold to a Northumberland bachelor along with a piece of land, she refuses to accept her plight. Instead this spirited ingenue (memorably played by Florence Pugh) schemes, sleeps and ultimately murders her way out of the patriarchal bodice. It is more Coen brothers thriller than Jane Austen romance.

But there is something else striking about Lady Macbeth: Katherine’s maid, Anna, is black. The cocksure groom Katherine takes as her lover is also dark-skinned. Two more major characters who appear late in the story are black. In fact, there are practically more characters of colour in Lady Macbeth than there are in all the Austens, Dickenses and Downtons put together. British period drama as a cordoned-off zone of whiteness may chime with current fantasies of a country unpopulated by immigrants or foreigners, but given that period drama accounts for such an overwhelming proportion of our entertainment industry, isn’t it a bit … old-fashioned?

There are two sides to this problem. First, the repercussions are being felt on Britain’s screens and stages as actors of colour are excluded. As Thandie Newton has said about being based in Britain: “I love being here, but I can’t work, because I can’t do Downton Abbey, can’t be in Victoria, can’t be in Call the Midwife … there just seems to be a desire for stuff about the royal family, stuff from the past, which is understandable, but it just makes it slim pickings for people of colour.”

As a result, many British actors – from Idris Elba to David Oyelowo to Chiwetel Ejiofor – go to the US to find work as well. Sophie Okonedo went to act with Denzel Washington on Broadway, Newton is currently in Westworld, Daniel Kaluuya jumped ship with American horror Get Out – and the list goes on. Samuel L Jackson recently questioned why all the plum Hollywood roles were going to Brits; if he channel-surfed on British TV, he would have seen why.

People tell me it wasn't like that. I say: 'How do you know? Really, how do you know?'

The second aspect of the problem is that these ethnically cleansed costume dramas give the impression that there were no people of colour in Britain’s past, which is far from the truth. In the third century, Roman emperor Septimius Severus, born in what is now Libya, brought his family and court to York. A fourth-century skeleton excavated in York in 2010 was found to have African characteristics and was buried with a bracelet made of ivory. DNA tests discovered rare African-specific chromosomes in white British men. Onyeka’s book Blackamoores documents the presence of Africans in Tudor Britain. Britain’s global empire and the legacy of slavery brought significant non-white populations to our shores.

The underrepresentation of black historical figures on screen has created a catch-22 situation. At the British Film Institute last year, Oyelowo spoke about trying to develop a movie based on Bill Richmond, a real‑life black bare-knuckle boxer in 19th‑century London. He read out one of his rejection letters, in which the producers explained that they couldn’t make his film since “a viewer must have a sense of what it is they are to get: either a familiar title or a piece of history that is ripe for a revisit”. In other words, even though this was a true story, the producers thought it would confuse audiences to see a black character in a period movie. Oyelowo went off to play Martin Luther King instead.

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Lady Macbeth’s director, William Oldroyd, observed a similar reaction to the actors of colour in his film: “People have said to me: ‘It wasn’t really like that in that period.’ And I say: ‘How do you know? Really, how do you know?’” His research suggested many well-to-do households of the era would have had black servants, and there was a significant population of African-descended Britons in the northeast in the 19th century. The region was more sympathetic to abolitionist causes than west-coast cities such as Liverpool or Bristol, which had vested interests in slavery. “That area of England was far more diverse than we have been led to believe. A lot of people make assumptions, and those assumptions are usually based on films they’ve seen already.”

Nobody remarks on anyone else’s ethnicity in Lady Macbeth, but the ethnic mix adds an extra element of class division to the story – it is only characters of the “servant class” who are played by actors of colour, and they are the victims of the piece. But the film’s casting wasn’t solely about making a race/class statement, or historical fidelity. “It was simply about putting the best actors in the best parts,” says casting director Shaheen Baig. So Anna, the servant, for example, was not cast as a black woman to make a point, but because the actor in question, Naomi Ackie, did such a good reading. It was the same with all the roles.

Baig, who has worked on screen projects from the ballet film Billy Elliot to the sci-fi TV series Black Mirror, casts colour-blind as a rule. “I don’t approach any project with the sense of ‘It has to be like this’, unless it is based on a real person or there is a genuine reason such as it’s absolute historical fact. Apart from that, I have no agenda.” Bigger budget period films, often with foreign money involved, tend to demand star names and a more stereotypical view of period Britain, she suggests; Lady Macbeth, with a budget of less than £500,000 had more freedom.

In addition, both Oldroyd and Baig question the notion of historical dramas being “realistic”. Characters in the 18th century didn’t have perfect teeth or complexions, their rooms weren’t clean or well lit, they didn’t speak with Rada accents.

Olivier played Othello in blackface in 1965, and Anthony Hopkins did the same for the BBC as recently as1980.But British theatre has travelled further down this road than cinema or television, and “colour-blind casting” has been the norm for decades on stage, with actors such as Oyelowo and Adrian Lester playing Shakespearean kings in the 2000s (Anglo-Indian Ben Kingsley played Hamlet back in 1975). On today’s British stage, a historical play with a multi-ethnic cast is entirely unremarkable. Critics are more likely to remark on the lack of diversity. The Royal Shakespeare Company’s 2012 production of Chinese play The Orphan of Zhao and the Print Room’s recent In the Depths of Dead Love were both criticised for telling east Asian stories with overwhelmingly white casts. It was the same with the production of Shakespeare’s TheWars of the Roses plays at the Rose theatre in Kingston in 2015, with an exclusively white cast. Director Trevor Nunn’s defence of “historical verisimilitude” was rejected by many critics. Why wasn’t Nunn hiring actors ridden with the pox, then? And why had he cast a Norwegian actor, Kåre Conradi, as a British king, asked the actor Danny Lee Wynter, founder of Act for Change, which has called for diversity quotas on stage and television.

When working in the theatre Oldroyd says he felt “far freer to basically say, we want to cast the best people for each part”, and it is often stage directors who have brought colour-blind casting into cinema. Kenneth Branagh, for example, put Denzel Washington in his 1993 film version of Much Ado About Nothing and has cast actors of colour in most of his subsequent movies, even Marvel’s Thor (in which Idris Elba as a Norse god ruffled some feathers) and his recent Disney Cinderella, which included Nonso Alonsie. Directors such as Dominic Cooke and Richard Eyre also embraced racial diversity in the BBC’s The Hollow Crown series of Shakespeare adaptations, featuring Okonedo as Queen Margaret.

Richard Briers, Robert Sean Leonard and Denzel Washington in Much Ado About Nothing (1993). Photograph: BFI

Does colour-blind casting really effect change, or is it merely a cosmetic improvement? Would a more substantial shift be to tell more stories about British people of colour? Some movies have done so. Amazing Grace, in 2006, centred on abolitionist William Wilberforce (played by Ioan Gruffud) but found only one speaking part for a black character (Youssou N’Dour as Olaudah Equiano). Amma Asante’s 2013 movie Belle – the true story of the illegitimate daughter of a British Royal Navy officer and an African slave in 18th-century London – at least put issues of race and a black woman (played by Gugu Mbatha-Raw) at the fore. Both films, however, viewed British slavery in terms of its abolition by a white saviour – and Belle takes considerable liberties with the real history to achieve it.

There must be plenty more of those stories out there. Stephen Frears is currently directing Victoria and Abdul, based on the true story of Queen Victoria’s friendship with a young Indian man, with Judi Dench and Ali Fazal in the lead roles.

A historical drama tells us more about the era in which it is made than the one it seeks to portray. “I think the genre is being called out for what it is,” says Oldroyd. “It’s typecasting actors and it’s typecasting genres. It’s lazy. This is not historical document we’re talking about: it’s fiction.”

Lady Macbeth is released in cinemas on 28 April.

This article was amended on 17 April 2017. An earlier version said incorrectly that The Wars of the Roses plays directed by Trevor Nunn in 2015 were produced by the RSC.